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Geologic Time

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					Geologic Time

  In a nut shell
       The Basics of Temporal Scale
• Relative Age Dating (chronostratic) - subdivisions of the Earth's geology in
  a specific order based upon relative age relationships (most commonly,
  vertical/stratigraphic position). These subdivisions are given names, most
  of which can be recognized globally, usually on the basis of index fossils.

• Absolute time (chronometric) - numerical ages in commonly obtained via
  radiometric dating methods performed igneous rocks (original crystal
  formation).
GEOLOGIC HISTORY AND TIME
                     FOSSILS
• EVIDENCE OF ONCE LIVING ORGANISMS PRESERVED IN
  ROCK OR OTHER MEDIA

• HELP TO UNDERSTAND EARTH’S DEVELOPMENTAL
  HISTORY


  REQUIREMENTS FOR FOSSIL PRESERVATION?
        RAPID BURIAL
        PROTECTION FROM SCAVENGERS
        HARD PARTS
        FRIENDLY ENVIRONMENT
          Types of Fossilization
•   Petrified Remains
•   Carbonaceous Films
•   Molds and Casts
•   Original Remains
•   Trace Fossils
Petrified Remains
• Some or all of the
  original materials in
  the remains have
  been replaced by
  minerals
   Film, Molds
    and Casts
• Carbonization – process
  of chemically changing
  material
• Mold – cavity in rock
• Cast – sediment filled
  cavity
                 Original Remains

• Actual organism or
  part found




Insects in amber
Woolly Mammoth in ice
La Brea tar pit, CA
            Trace Fossils

• Evidence of animal
  activity



Animal tracks (footprints)
Worm holes and burrows
                  Index Fossils

•    Used to determine the age of rock layers via
     correlation of strata.

    1. Existed for relatively short period of time
    2. Abundant
    3. Widespread geographically
Examples of Index Fossils
    INTERPRETING GEOLOGIC HISTORY
• GEOLOGIC HISTORY IS RECORDED IN THE ROCK
  RECORD – OBSERVATIONS OF COMPOSITION,
  STRUCTURE, POSITION, AND FOSSIL CONTENT
  LEADS TO INTERPRETATIONS OF THE GEOLOGIC
  HISTORY OF EARTH.

  – RELATIVE AGE DATING OF ROCKS AND EVENTS
     • Used to determine the order of events and relative
       age of rocks by examining the position of rocks in a
       sequence
     • UNIFORMITARIANISM
     RULES FOR RELATIVE AGE DATING

• PRINCIPLE OF SUPERPOSITION
  – OLDEST ROCKS ON BOTTOM
• PRINCIPLE OF ORIGINAL HORIZONTALITY
  – DEPOSITED AS HORIZONTAL LAYERS OF SEDIMENTS
• PRINCIPLE OF INCLUSIONS
  – ROCK MUST BE OLDER THAN LAYER FOUND IN
• PRINCIPLE OF CROSS-CUTTING RELATIONS
  – GEOLOGIC FEATURES YOUNGER THAN LAYER IT CUT
    • EXAMPLES?
Igneous Intrusion Examples
Principle of Cross-Cutting Relations
     UNCONFORMITY
•   Gaps in the rock record that
    develop when agents of
    erosion remove existing rock
    layers

    Three types
       1. Angular Unconformity
       2. Disconformity
       3. Nonconformity
      1. Angular Unconformity
• Tilted layers of rock with horizontal layers
  deposited on top
            2. Disconformity
• All layers are horizontal, however, erosional
  surfaces found where rocks were exposed and
  eroded.
           3. Nonconformity
• Form when sedimentary rock layers form above
  metamorphic or intrusive igneous rock
          Correlating Rock Layers




Match up or correlate layers of rock over great distances
                 ABSOLUTE AGE DATING
= A METHOD USED TO DETERMINE THE AGE, IN YEARS, OF A ROCK OR
OTHER OBJECT



  HALF-LIFE = THE TIME IT TAKES FOR HALF OF THE ATOMS IN THE
  ISOTOPE TO DECAY




  •HALF-LIFE TIME IS DIFFERENT FOR EACH TYPE OF RADIOACTIVE ISOTOPE

  •ALWAYS THE SAME FOR A GIVEN ISOTOPE
      •Not affected by Mass,   Temperature, Pressure
RADIOACTIVE DECAY CURVE
PARENT AND DAUGHTER GRAPHS
                 GEOLOGIC TIME
• Division of Earth’s history into smaller
  units based on the appearance or
  disappearance of types of organisms
  and on geologic change
• Subdivisions include;
   – Eras
       • Major subdivisions based on differences in
         life-forms… mass extinctions
   – Periods
       • Based on types of life existing at the time
         and on geologic events
   – Epochs
GEOLOGIC TIMELINE
               Resources:
• http://www.ucmp.berkeley.edu/help/timefor
  m.html

				
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